a. Field of the Invention
The present invention relates to a thermosetting powder paint composition excellent in weather resistance as well as in impact resistance and chipping resistance.
b. Description of the Prior Art
[Trend of Research and Development in the Technical Field of Paint and Expectation for Powder Type Paint from the Viewpoint of Ecological Point]
In recent years, expectations for powder type paints have been growing larger in place of solvent type paints in the technical field of paints from the viewpoint of local or global preservation of the environment, occupational safety and health and environmental improvement, prevention of fire and explosion, saving of resources, and the like.
Further, as expectations have been growing larger for highly functionalized and diversified powder type paints by historical and social requests, there have been increasing demands for powder type paints having high film performances (e.g. impact resistance, resistance to acid rain, etc.) comparable with solvent type paints. However, although film performances demanded for powder type paints have become strict, powder type paints entirely meeting such demands have not been available on the market.
Illustrative examples of conventional powder paints may embrace, for example, epoxy resin and polyester resin powder paints prepared primarily from bisphenol A. However, these paints have had problems not only in weather resistance but also in resistance to acid rain which has recently and particularly become a problem in keeping with the acidification of the environment. Therefore, the powder paints have problems in applications premised on outdoor use such as car body coating.
To overcome these defects, an acrylic powder paint was proposed in Japanese Patent Publication No. 38617/1973 to achieve a remarkable improvement in weather resistance. However, the acrylic powder paint is also inferior to conventional polyester powder paints in impact resistance.
In other words, powder type paints satisfying all of weather resistance, resistance to acid rain and impact resistance were not available on the market. Thus, powder type paints satisfying all of weather resistance, resistance to acid rain and impact resistance have been demanded from the market, and research and development have been made energetically so as to market such paints.
[Prior Art Relating to Improvement in Impact Resistance of Acrylic Powder Paint Films]
In such circumstances of development in the impact resistance of the acrylic powder paint films, techniques of below (1) to (3) by way of example have been disclosed. However, it cannot be mentioned yet that any decisive solution have been found. For instance, improvements were attempted in low-temperature curability and recoating properties, (1) by mixing a dibasic acid, etc. (Japanese Patent Laid-Open No. 112743/1993) and (2) by mixing an alkyl titanate in the technique of (Japanese Patent Laid-Open No. 165463/1988), respectively. In both the techniques, however, impact resistance was mentioned only as a secondary effect which was insufficient.
(1) Japanese Patent Laid-Open No. 112743/1993:
In Japanese Patent Laid-Open No. 112743/1993 (title: Pulverulent Coating Composition, applicant: Hoechst AG), a technique is disclosed to obtain a pulverulent coating composition excellent in curability.
Specifically, there is disclosed a technique concerning a pulverulent coating composition comprising (A) a copolymer having glycidyl groups, (B) an aliphatic or alicyclic dibasic acid, an anhydride thereof or a polyol-modified anhydride of the dibasic acid, and (C) a pigment and other additives as required.
Here, the copolymer (A) has a number average molecular weight (Mn) of 1,000 to 10,000 and a glass transition temperature of 30 to 90.degree. C. and comprises (a) at least 20% by weight of acrylate or glycidyl methacrylate, (b) 35 to 50% by weight of styrene, (c) 10 to 45% by weight of one or more alkyl esters of an aliphatically unsaturated monocarboxylic acid or dicarboxylic acid, and (d) 0 to 50% by weight of one or more other olefinically unsaturated monomers.
In this technique, however, impact resistance was mentioned only as a secondary effect which was insufficient.
(2) Japanese Patent Laid-Open No. 165463/1988:
In Japanese Patent Laid-Open No. 165463/1988 (title: Thermosetting Acrylic Resin Powder Paint Composition, applicant: Kansai Paint Co., Ltd.), a technique is disclosed regarding a thermosetting acrylic resin powder paint composition which is composed principally of a specific glycidyl-functional acrylic resin, an aliphatic dibasic acid (anhydride) and an alkyl titanate compound. The composition melts and cures at low temperatures, and moreover gives paint films excellent in hardness, impact resistance, flexing resistance and the like.
Specifically, the technique concerns a thermosetting acrylic resin powder paint composition which is composed mainly of (A) a glycidyl-functional acrylic resin obtained by copolymerizing primarily 1-14 carbon alkyl ester of (meth)acrylic acid and a glycidyl ester of (meth)acrylic acid, (B) an aliphatic dibasic acid (preferably adipic acid, sebacic acid, decandioic acid, muconic acid, etc.) or an anhydride thereof, and (C) an alkyl titanate compound represented by formula Ti(OR).sub.4 (wherein R is an alkyl group having 15 to 20 carbon atoms) (e.g. tetrapentadecyl titanate, etc.).
In this technique, however, impact resistance was mentioned only as a secondary effect which was insufficient.
(3) Japanese Patent Publication No. 25709/1987:
In Japanese Patent Laid-Open No. 230068/1984 (Japanese Patent Publication No. 25709/1987, Registration No. 1417360, title: Novel Epoxy Resin Composition for Powder Paint, applicant: Asahi Chemical Industry Co., Ltd.), a technique is disclosed regarding a novel epoxy resin composition for a powder paint which comprises an epoxy resin and a specific amount of a specific epoxidized rubber, is excellent in impact resistance, flexibility and corrosion resistance, and does not lose impact absorbing capacity even at low temperatures.
Specifically, there is disclosed a technique concerning a novel epoxy resin composition for a powder paint which comprises (A) 97 to 65 parts (by weight) of an epoxy resin having two or more epoxy groups in the molecule and an epoxy equivalent of 400 to 2,500 and (B) 3 to 35 parts of a rubber component of an epoxidized rubber formed by grafting (1) base layer rubber particles having a toluene-insoluble gel content of 50% or more with (2) a polymerizable epoxy compound having 0.5 or more epoxy group and 0.1 to 1.0 acryloyl group per molecule and/or a monoethylenic glycidyl ester or ether and (3) an ethylenic monomer.
Then, the composing elements, the action and effect of the above technique are summarized below.
i) Resin Component:
The resin component used in the above technique is an "epoxy resin". The concept of the term "epoxy resin" used in the above technique textually includes not only epoxy resins in a narrow sense (epoxy resins of bisphenol A diglycidyl ether base) but also epoxy resins in a broad sense (resins having two or more epoxy groups in the molecule). Here, the epoxy resins in a narrow sense are epoxy resins of bisphenol A diglycidyl ether base which have industrially the largest, overwhelming share and are commonly referred simply to as "epoxy resin" as a usual practice by persons skilled in the art.
However, only epoxy resins in a narrow sense are mentioned in all the descriptions including the best mode as the epoxy resins of the above technique. Hence, no disclosure is made on epoxy resins in a broad sense except the above epoxy resins in a narrow sense.
ii) Curing Agent Component:
The curing agent used in the above technique is not particularly limited. Specific examples of the curing agent used in the above technique include novolak phenol resins, dicyandiamide, imidazoles, hydrazides, aromatic amines and acid anhydrides.
iii) Rubber Particles:
In the above technique, rubber particles having specific conditions are one of the composing elements of the invention in order to improve the impact resistance of paint films by forming the paint films having an ocean-island structure consisting of a continuous phase (ocean) of a resin crosslinked by a curing agent and a dispersed phase (islands) of the rubber particles.
The rubber particles having specific conditions are formed by introducing epoxy groups or glycidyl groups into the surface of the particles.
The reason for introducing a specific amount of epoxy group into the surface of rubber particles is conceivably to improve the compatibility or interfacial adhesion between an ocean and islands, because in a multiphase polymer matrix having an ocean-island structure, the interfacial energy or compatibility between an ocean and islands is important in order to secure the uniformity of the matrix.
iv) Action, Effect and Applications:
In the above technique, illustrative examples of its applications include external painting of steel pipes, coating of steel bars, electrical insulation and paints, for which a high impact resistance is demanded.
(4) Difference between Japanese Patent Publication No. 25709/1987 and the present invention:
The difference between Japanese Patent Publication No. 25709/1987 and the present invention is illustrated hereunder.
i) Resin Component:
The resin component used in the above technique is a "epoxy resin". The present inventors have examined zealously paint films which are hit by gravels and pebbles at a high relative speed during traveling and exposed to the sunlight and severe weather conditions in the open air without any shelters, like those on the surface of bodies and chassis of vehicles such as automobiles. As a result, it was confirmed that the best mode of the above technique were inferior in chipping resistance, weather resistance and appearance.
Judging from the disclosure regarding the applications of the above technique, it is thought that the inventors of the above technique did not expect such applications of paint films as on the surface of bodies and chassis of vehicles such as automobiles mentioned above. Namely, it is assumed that they expected the applications indoors or outdoors with shelters.
Separately, the resin component used in the present invention is an acrylic or methacrylic resin having two or more epoxy groups in the molecule, which exhibits an excellent weather resistance even in such applications.
Namely, the resin component used in the present invention contributes greatly to exhibit excellence in appearance, impact resistance and chipping resistance in the constitution of the present invention when it is hit by gravels and pebbles at a high relative speed during traveling and is exposed to the sunlight and severe weather conditions in the open air without shelters, in the applications for the surface of bodies and chassis of vehicles such as automobiles in which excellent appearances (smoothness, clear brilliance, etc.) are demanded.
As mentioned above, the resin component used in the present invention is textually included in the concept of the epoxy resin defined in the above technique. However, it is not disclosed concretely in that. Therefore, from the viewpoint of the action and effect of weather resistance including ultraviolet light resistance and sunlight resistance, the resin component used in the present invention is a lower concept which is selected from the concept of the epoxy resin defined in the above technique. The lower concept is not disclosed concretely in the above technique and gives remarkable unexpected effects by being combined with the other components.
ii) Curing Agent Component:
The curing agent used in the above technique is not particularly limited.
The present inventors have examined zealously paint films which are hit by gravels and pebbles at a high relative speed during traveling and exposed to the sunlight and severe weather conditions in the open air without any shelters, like those on the surface of bodies and chassis of vehicles such as automobiles. As a result, it was confirmed that paint films, in which the composing element of rubber particles was excluded from the best mode of the above technique, were inferior in impact resistance, weather resistance and appearance.
On the other hand, the curing agent used in the present invention is at least one polybasic carboxylic compound (B) selected from the group consisting of poly-basic carboxylic acids (b1) and polybasic carboxylic anhydrides (b2). Specific examples thereof are described below.
The curing component used in the present invention contributes greatly to exhibit excellence in appearance, impact resistance and chipping resistance in the constitution of the present invention when it is hit by gravels and pebbles at a high relative speed during traveling and is exposed to the sunlight and severe weather conditions in the open air without shelters, in the applications for the surface of bodies and chassis of vehicles such as automobiles in which excellent appearances (smoothness, clear brilliance, etc.) are demanded.
As mentioned above, the curing agent component used in the present invention is textually included in the concept of the curing agent of the above technique. However, from the viewpoint of giving unexpectedly remarkable action and effect in impact resistance and chipping resistance by selecting the specific curing agent and combining it with the other specific resin component and rubber particles, the curing agent component used in the present invention is a lower concept specifically selected from the concept of the curing agent in the above technique.
iii) Rubber Particles:
In the above technique, the rubber particles wherein a specific amount of epoxy group (0.5 or more epoxy group per molecule of the polymer) is introduced into the surface of the particles, are one of composing elements of the invention.
The reason for introducing a specific amount of epoxy group into the surface of rubber particles is conceivably to improve the compatibility or interfacial adhesion between an ocean and islands in a multiphase polymer matrix having an ocean-island structure, as mentioned above.
The present inventors have examined zealously paint films which are hit by gravels and pebbles at a high relative speed during traveling and exposed to the sunlight and severe weather conditions in the open air without any shelters, like those on the surface of bodies and chassis of vehicles such as automobiles. As a result, it was confirmed that although the best mode of the above technique was superior in impact resistance, it was inferior in both chipping resistance and film appearance.
In the above technique, epoxy group which is a common functional group for an ocean and islands is arranged at high densities in both the ocean and the islands in order to improve the compatibility or interfacial adhesion between the ocean and the islands. Here, the present inventors have thought that the crosslinking density between an ocean and islands is therefore too high and the super local elastic modulus around islands is also too high, so that the paint film can not absorb local impacts completely and hence has an inferior chipping resistance (local impact resistance).
Separately, the rubber particles used in the present invention do not need such introduction of epoxy group into the surface of the particles. In the present invention, the particles (C) having a core/shell structure which constitute a dispersed phase (islands) can realize an uniform multiphase polymer system by selecting a monomer whose polymer has a solubility parameter (SP value) near that of an acrylic or methacrylic resin constituting a continuous phase (ocean) as monomer units in the molecule of a shell (particle surface) polymer or by introducing a suitable compatibilizer as a third component, without introducing epoxy or glycidyl group entirely or at a high density into the surface of the particles.
The particle component used in the present invention contributes greatly to exhibit not only excellent impact resistance but also excellent chipping resistance and excellent appearance in the constitution of the present invention when it is hit by gravels and pebbles at a high relative speed during traveling and is exposed to the sunlight and severe weather conditions in the open air without shelters, in the applications for the surface of bodies and chassis of vehicles such as automobiles, in which excellent appearances (smoothness, clear brilliance, etc.) are demanded.
Therefore, the particles having a core/shell structure used in the present invention are not included in the rubber particles of the above technique.
iv) Action, Effect and Applications:
In the above technique, illustrative examples of its applications include external painting of steel pipes, coating of steel bars, electrical insulation and paints, for which a high impact resistance is demanded. The above technique discloses an improvement in the impact resistance of paint films particularly at low temperatures as the action and effect of the invention.
It is thought as described above that the inventors of the above technique do not expect applications under such conditions that the coating surface is hit by gravels and pebbles at a high relative speed during traveling or exposed to the sunlight and severe weather conditions in the open air without shelters, like the surface of bodies and chassis of vehicles such as automobiles.
On the other hand, the action and effect of the present invention is to realize concurrently excellent weather resistance, chipping resistance, impact resistance and external characteristics, which are difficult to obtain by the paint film of the above technique.
Accordingly, the present invention can be applied very preferably to paint films in the foregoing applications in which the above technique is hard to employ.
v) Difference between Japanese Patent Publication No. 25709/1987 and the present invention:
As is obvious from the above 1) to iv), Japanese Patent Publication No. 25709/1987 is different from the present invention in the composing elements and action and effect.
Namely, by adopting a constitution different from that of the above technique, the present invention can be applied very favorably to paint films which are hit by gravels and pebbles at a high relative speed during traveling or exposed to the sunlight and severe weather conditions in the open air without shelters, like those on the surface of bodies and chassis of vehicles such as automobiles, to which the above technique is hard to apply.
[Chipping Resistance]
The concept of the term "chipping" used in the claims or the specification of the present application includes the phenomenon of impact fracture upon loading on a pin-point in a short time, and particularly in the technical field of car paints, also includes the phenomenon of flawing which car body paint films suffer upon hitting flying pebbles.
The concept of the term "chipping resistance" used in the claims or the specification of the present application embraces the resistance of paint films to "chipping".
Specific examples of the method for evaluation of chipping resistance include, for example, the scattering stone test by a gravellometer according to SAE-J400 and ASTM D-370 employed in the United States which are evaluation methods for paint films for automobiles. In these evaluation methods, chipping resistance is determined by hitting pebbles of a given particle size against a paint film by a given force to evaluate the diameter of thereby caused flaws in the paint film.
[Impact Resistance]
The concept of the term "impact resistance" used in the claims or the specification of the present application includes the phenomenon of impact fracture upon loading over a wide area in a short time and, particularly in the technical field of car paints, also includes the phenomenon of flawing which car body paint films sustain upon hitting a large object.
Illustrative examples of the method for evaluating impact resistance may embrace, for example, the impact resistance test (Du Pont impact test) according to JIS K5400 6. 13. 3. In such an evaluation test, impact resistance is judged by evaluating a drop height at which cracking and scaling occur in a paint film when a weight (500 g or 1 kg) is dropped onto the paint film.
[Correlation Between Impact Resistance and Chipping Resistance]
Formerly, the difference of concept between impact resistance and chipping resistance of paint films has scarcely been recognized in the technical field of powder paints. Further, little attention has been paid to the importance of the chipping resistance of paint films, though the impact resistance of paint films has been regarded as important. In other words, although the impact resistance of paint films has been evaluated, the chipping resistance of paint films has hardly been evaluated. Further, no investigation has been made on the correlation between impact resistance and chipping resistance. The excellence in impact resistance did not necessarily mean excellence in chipping resistance.
For instance, in Japanese Patent Laid-Open No. 221567/1991, a technique is disclosed wherein a powder paint film is cured by an acid/isocyanate reaction to improve the impact resistance of the paint film. Even in the Japanese Patent Laid-Open No. 221567/1991, the concept of the chipping resistance of paint films is not thought of at all. Further, only the impact resistance of paint films is regarded as important, but no attention is paid at all to the importance of the chipping resistance of paint films.
From such backgrounds, the present inventors have noticed the correlation between impact resistance and chipping resistance of a paint film formed by a powder paint.
For example, U.S. Pat. No. 3,845,016 (Santokh S. Labana et al.), U.S. Pat. No. 3,919,347 (Themistoklis Katsimbas et al.), Japanese Patent Laid-Open No. 112743/1993, etc. describe that powder paint films cured by a glycidyl group/acid anhydride group reaction are inferior in impact resistance.
The present inventors tested the paint films disclosed therein on the impact resistance. Then, these films were found definitely to be inferior in impact resistance. At the same time, the present inventors individually tested on the chipping resistance. Then, it was confirmed that in spite of inferiority in impact resistance, the films had a superior chipping resistance conversely.
Thus, it was affirmed that there was no correlation between impact resistance and chipping resistance so far as at least these cases were concerned.
The present inventors have taken up as a problem to be solved imparting not only an impact resistance but also a chipping resistance to paint films formed by a powder paint, based on this result and noting the fact that persons skilled in the art have taken up impact resistance as a problem to be solved but have hardly taken up chipping resistance as the problem in the prior art of powder paints including acrylic ones.
From such points of view, the present inventors have proceeded with an intensive investigation based on an absolutely new technical idea. The new technical idea is to apply the concept of the toughening mechanism of a microphase separated structure in the technical field of "polymer alloy" or "polymer blend" in the region of the polymer chemistry into the technical field of powder paints, so that imparting simultaneously an impact resistance and chipping resistance to paint films.
In contrast to the prior art, the present invention has a novelty in that an excellent impact resistance and chipping resistance are granted to the whole paint film owing to the impact resistance of a particles having a core/shell structure. The particles having a core/shell structure are formed by arranging a polymer having a calculated glass transition point of 20.degree. C. or below in the core and a polymer having a calculated glass transition point of 40.degree. C. or above in the shell, and the particles are uniformly held or contained as a dispersed phase (islands) in a continuous phase (ocean) comprising a organic polymer by the three dimensional crosslinked network structure or IPN (interpenetrating network) structure of the organic polymer.